Materials for gas turbine power generation
At the end of 2007 Materials UK launched a comprehensive set of reports on materials for energy. Colin Small of Rolls-Royce plc explores the Fossil Fuelled Power Generation Report, concentrating on gas turbine power generation.
Current UK plans to meet heightened demand for energy, low emissions and cost effective electrical power centre on a mixed approach – renewables, nuclear power and fossil fuel (coal and gas) plant. In the UK there are currently around three gigawatts of new gas fired combined cycle turbine generating capacity under construction, with an additional one gigawatt approved and a further six gigawatts planned. Gas turbines will thus be a major element of future UK power generation and are one of the most versatile, flexible and efficient forms of power generation.
The success of the gas turbine power generation industry in the UK is dependent upon satisfying customer and operator requirements, while simultaneously optimising the complex technical challenges arising from market and legislative factors. These affect capital and operating costs, efficiency, power output, fuel choice and emissions. The key driver in building fossil fuel fired gas turbine plants is thus to maximise the power generated while minimising the impact on the environment at an economically viable cost. The main approach to do this is therefore to increase plant efficiency through higher temperature duty cycles/materials and improved designs.
More efficient designs that incorporate hardware such as reformers, intercoolers, recuperators and heat exchangers are being developed, but they lead to more complex plant and harsher running conditions for core components.
The use of gasification or renewable, CO2 neutral, fuels can also help reduce emissions. A number of potential fuels and manufacturing processes have been identified. However, these fuels tend to be of lower calorific value which introduces several new problems:
• Increased fuel mass flow for a given heat input requirement.
• Fuel-borne nitrogen leading to higher NOx emissions.
• Increased fuel gas temperature and higher levels of contaminants.
Current estimates are that these design enhancements will lead to efficiency improvements of 15-20% over current levels. For a 500MW gas-fired station, this equates to a reduction in CO2 emissions from 0.36Mt/year to 0.29Mt/year.
However, none of this improvement can be achieved without suitable materials. We therefore need to develop new generations of advanced high temperature materials.
A holistic approach
There are five key technologies that are essential to ensure that a viable materials systems solution is delivered from a materials development programme:
• Surface protection (coatings).
• Improved understanding and predictive modelling of degradation mechanisms (lifing).
• Non-invasive inspection techniques for in situ assessment of the material (non-destructive evaluation).
• Component refurbishment (repair).
• Similar and dissimilar materials joints (joining).
These technologies must be integrated into the materials and manufacturing development programme to ensure the solution delivered is viable and has the necessary supporting technologies required to deploy it. The solution must also be useable on an industrial scale, so development with supply chain involvement is also necessary. It is essential that all future materials programmes develop answers on this holistic basis.
In the short to medium term (five to 10 years) the materials solutions are under development, or have been identified, based on known technology and involve incremental changes to existing materials systems. This is because of the long development times needed to deploy a solution on an industrial scale and partially because current designs are limited to known materials. However, much of the current high temperature materials technology is derived from aero-engine developments. With the increasingly divergent modes of operation for energy and aero operation, and the escalating cost of aero technology, more cost effective, tailored solutions for energy-generating gas turbines are required. The main materials developments required over this timescale are:
• Incremental development of existing advanced high temperature material systems but tailored for industrial gas turbine applications. This covers all classes of materials, components and their coating systems.
• Advanced manufacturing development for cost reduction.
• Increased materials performance, and improved integrity, including materials and process modelling.
Over the 20-year timescale there is an urgent need to launch and sustain the ‘blue skies’ research to identify the step change ‘disruptive materials’ technology that will provide a new generation of materials for ultra high efficiency designs.
These designs will have a major impact on reducing emissions and cost of electricity in the long term and will achieve the 15-20% improvement in efficiency predicted. But, even with advance materials and process modelling, to reduce the costs and timescales, these will take 15-20 years to develop as the potential solution has not yet been identified and we do not know what it is or how it will be manufactured. This is therefore the secondary priority to the five to 10 year needs, but one that cannot be ignored.